1. Field of the Invention:
The present invention relates to catalytic compositions and methods for their preparation and use. More particularly, this invention relates to a catalyst undercoat material useful for providing an undercoat or alternatively a support having a catalytically advantageous pore size distribution and high porosity.
2. Description of the Prior Art:
There are many types of wash-coats or undercoats (hereinafter referred to as coatings) as well as solid prefabricated catalyst foundations known in the art. It should be noted at the outset that the word support as used in this application applies primarily to low surface area structures which serve as catalyst foundations. To this low surface area support is applied an undercoat or wash-coat having relatively high surface area which serves as an anchor or base for the catalytic entity forming the outermost surface of the catalytic composite. Alternatively, the catalyst undercoat material may be compressed or extruded into various geometric forms and used directly to support a catalyst. Such a solid may be in a variety of forms including powders, granules, sheets, spheres, extrudates, honeycombs or monolith structures, cylinders or rings, saddles, stars and the like. An example of a material serving as both a wash-coat and a high surface area catalyst support is alumina which is used widely as an undercoat and equally widely in the form of spheres, cylinders and extrudates of various configurations as a high surface area support. The undercoat can be thought of as a film of high surface area applied to a low surface area support to attain high catalytic activity not otherwise attainable with low surface area supports.
A variety of high strength low pressure drop catalyst supports are known in the art. These supports characteristically have a smooth surface and are catalytically inert, non-porous and have a low surface area.
Before a catalyst can be applied to the surface of the support material a film or layer of high surface area material must first be applied Such multi-layer catalysts are frequently used in the chemical industry or in abatement processes for the disposal of combustible or toxic materials including reducing pollutants in waste gases. For example, these catalysts may be used for the oxidation of carbonaceous materials as well as the reduction of nitrogen oxides contained in automobile exhaust.
The current art can be illustrated by the typical catalytic composite comprising a sturdy foundation illustrated by a low surface area honeycomb or similar monolithic support. Because this foundation material typically has a smooth surface and is dense and non-porous, a film or coating of a strongly adherent, refractory, high surface area and porous nature is applied. It is to this coating that our invention is directed and will subsequently be fully described. Finally onto and into this film is applied by impregnation, immersion, spraying or other means the catalytic coating comprising oxidation catalysts from the precious or base metal groups.
Examples of such prior art catalytic composites include those disclosed in U.S. Pat. No. 3,993,572 wherein the catalyst component contains a platinum group metal, a rare earth metal and an alumina component. The rare earth metal oxides disclosed include cerium, samarium and praseodymium. The catalyst component may be prepared by co-precipitating the ingredients. Such co-precipitation is described as either surface absorption or precipitation of a liquid or solid. The impregnation of alumina powder with cerium salts is also disclosed.
U.S. Pat. No. 3,867,309 relates to the deposition of palladium and a metal selected from the group consisting of a rare earth, iron, manganese and zinc to high surface area gamma alumina spheres. An example of a rare earth or lanthanide is cerium nitrate. Similarly U.S. Pat. No. 3,903,020 describes the impregnation of cerium onto and into already formed alumina particles, generally in the form of spheres. The cerium salt is preferably cerium nitrate. After applying the cerium nitrate to the surface of the spheres, the entire composite is then calcined to decompose the lanthanide nitrate and introduce the interspersed oxide as a stabilizing agent into the alumina.
U.S. Pat. Nos. 3,867,312 and 3,899,444 disclose another procedure which includes preparing an aqueous solution of water soluble decomposable salts of a rare earth metal and a water soluble aluminum salt, evaporating the free water from the solution and then heating the resultant mixture of aluminum salt and rare earth metal salt. A uniform mixture of salts is obtained and subsequently decomposed to relatively low surface area oxides. The mixed oxides are cooled and a support material is thus produced. This procedure and slight variations have found wide application because the decomposition of the nitrate has the desirable result of producing a comparatively adhesive oxide or mixed oxide. However, the activity of such oxides is very poor because the decomposition procedure results in an oxide which as noted above is both dense and low in surface area. Further, these patents are primarily concerned with the adhesion of the film and only to a lesser extent, the other physical characteristics. U.S. Pat. No. 3,867,312 refers to the formation of a gel from the precipitation of lanthanum and aluminum hydroxide which gel is then processed into spheres. These spheres become the support material for the catalytic coating. No reference is made to the use of this product as an adhesive film.
While most of the activity in the design of chemical reactions or of catalytic abatement processes relates to the development of catalysts, it has become increasingly apparent that the catalytic support material is itself an important factor in the overall design of the catalyst and the operating facilities. The catalyst and catalyst bed must be prepared in such a manner as to minimize pressure drop. The current designers have recognized that in order to obtain maximum catalyst performance, the catalyst support or catalyst support surface must be among other things rugged and of such a design that the gases or liquids to be reacted will pass through and contact the catalyst deposited on the surface of the catalyst support material without allowing a substantial pressure drop.
Not only is it necessary that the contact be intimate, but it is also essential that the catalyst that is applied to the catalyst support has a high inherent activity, even when the catalyst is present as a thin film on the nonporous surface of a rugged support. This requirement translates into a catalytic film having a specific chemical composition with a catalytically advantageous pore size distribution, high stability, high porosity and firm adherence to the smooth surface low pressure drop support.
Thus, while the art has generally recognized the use of a lanthanide for the stabilization of alumina, there remains a need for highly stable catalytic support compositions with novel control of total pore volume, pore distribution and thermal stability assuring retention of these characteristics.
The above cited patents are generally representative of the state of the art. From these teachings, it becomes readily apparent that in general a water soluble lanthanide salt or salts are added to some form of particulate alumina which is later further processed and ultimately applied as a coating or made into a support in the form of spheres, cylinders, extruded rods and the like. In such preparations, the intimate mixture, high porosity and reproducible physical characteristics taught by the present invention are completely missed because either the particulate alumina or lanthanide defeats the need for co-precipitation herein taught. For example, it is impossible to derive an intimate or integral mixture in unit crystal to unit crystal relationships by the teachings of the prior art because the crystals in the particulate portion are massive relative to the truly co-precipitated materials formed by the practice of the instant invention.